Coated articles find application in many varied environments including, but not limited to, aerospace, automotive, marine, oil, gas and chemical engineering, electronics, medicine, robotics, textile and other industries. A useful but non-limiting application of coated articles is for coated valve metals (e.g., barrier layer-forming metals or rectifier metals) and their alloys, such as aluminum, magnesium, titanium and alloys thereof, which are widely used in different industries. To improve the valve, valve component, or valve surface wear resistance, chemical resistance and dielectric strength, for example, a protective coating possessing the necessary properties for a desired application may be formed on a respective surface or surfaces thereof. Various conventional anodizing processes can provide some of these protective properties. In a typical aluminum anodizing process, an aluminum article is placed in a bath containing an electrolyte, such as sulfuric acid, and an electric current is passed through the aluminum article (i.e., anode). Due to electrolytic oxidation, a protective aluminum oxide layer forms on the surface of the aluminum article. The resulting finish is extremely hard and durable, and exhibits a porous structure which allows secondary infusions, such as lubricity aids.
Conventional anodizing processes include, for example, U.S. Pat. Nos. 3,956,080, 4,082,626, and 4,659,440 which disclose methods of coating aluminum and other valve metals and their alloys by an anode spark discharge technique using direct current with voltages up to 450 V and current densities 2-20 A/dm2, usually around 5 A/dm2. The properties of the ceramic coatings are dependent on the composition of the electrolytic solutions, as well as on other process conditions, such as temperature, current voltage and density. Generally, it is possible to form ceramic coatings with good corrosion and chemical resistance; however, their mechanical properties, such as hardness, durability, and adherence to the substrate, are not entirely satisfactory. Moreover, the coating rate is relatively slow and, thus, productivity is limited.
In other conventional processes, such as described in U.S. Pat. Nos. 5,147,515 and 5,385,662, high voltage direct current of different waveforms is used, with voltages of about 1,000 V and even up to 2,000 V. These ceramic coatings have much better mechanical properties, such as hardness. However, their thicknesses are limited to about 80 μm and 150 μm, respectively. The rate of coating deposition is also relatively slow, at best reaching 1.75 μm/min, usually around 1 μm/min. The necessity of using high voltages and current densities (between 5-20 A/dm2) makes the process very energy consuming and, thus, expensive. In addition, the process described by Kurze et al. (U.S. Pat. No. 5,385,662) requires bath temperatures in the range of −10° to +15° C. and only allows for a very narrow temperature fluctuation range of ±2° C. It is also not clear how the different shapes of current affect the coating properties.
In U.S. Pat. Nos. 5,616,229 and 6,365,028, a high voltage (at least 700 V) alternating current is used instead of direct current. The thus-formed ceramic coatings have very good mechanical properties, with hardness exceeding 2,000 HV and adhesion to the substrate up to 380 MPa. The coating deposition rate is in the range 1-2.5 μm/min, which also compares favorably with previous methods. The method described in U.S. Pat. No. 5,616,229 uses a high voltage alternating current power source with a special, modified wave form, obtained by using a capacitor bank connected in series between the high voltage source and metal which is being coated. Although the disclosed method enables the formation of relatively thick coatings at a high rate of deposition, it is not clear how the current wave form is preserved and controlled during the process of ceramic deposition and how a possible departure from that wave form influences the process. Moreover, the proposed apparatus has a complex design due to the use of several baths containing different electrolyte solutions in which components are being coated in sequence. Power demands are still very high in both methods (the method described in U.S. Pat. No. 6,365,028 requires in its initial stage a current density of 160-180 A/dm2), and it is not clear if components of low thickness, e.g. 50 μm or less, and components of complex shapes with uneven residual (locked-up) stresses or of large surface sizes can be coated. U.S. Publication Application No. 20020112962 A1, published Aug. 22, 2002, is generally similar to the aforementioned patents and teaches optimization of current and voltage during various stages of coating.
All the aforementioned patents and described therein methods of forming ceramic coatings differ from one another in either the type of current used (DC or pulse DC or AC), voltage and current density values, or specific current wave forms and generally assign a significant role to the composition of electrolyte solutions. However, specific electrolytes are often very similar and vary with respect to only a pair of ingredients.
Accordingly, there is a need for an improved process for forming a ceramic coating on an article that addresses the disadvantages present in the known processes and for composite articles comprising an improved ceramic coating.